In a typical cellular radio system, wireless terminals, also known as mobile stations and/or user equipments (UEs), communicate via a Radio Access Network (RAN) to one or more core networks. The radio access network covers a geographical area which is divided into cell areas, with each cell area being served by a base station, e.g., a Radio Base Station (RBS), which in some networks may also be called, for example, a “NodeB” in a Universal Mobile Telecommunications System (UMTS) or “eNodeB” (eNB) in a Long Term Evolution (LTE) network. A cell is a geographical area where radio coverage is provided by the radio base station equipment at a base station site. Each cell is identified by an identity within the local radio area, which is broadcast in the cell. Another identity identifying the cell uniquely in the whole mobile network is also broadcasted in the cell. The base stations communicate over the air interface operating on radio frequencies with the user equipments within range of the base stations.
In some versions of the RAN, several base stations are typically connected, e.g., by landlines or microwave, to a controller node, such as a Radio Network Controller (RNC) or a Base Station Controller (BSC), which supervises and coordinates various activities of the plural base stations connected thereto. The RNCs are typically connected to one or more core networks.
A UMTS is a third generation mobile communication system, which evolved from the second generation (2G) Global System for Mobile Communications (GSM). The UMTS terrestrial radio access network (UTRAN) is essentially a RAN using Wideband Code Division Multiple Access (WCDMA) for user equipments. In a forum known as the Third Generation Partnership Project (3GPP), telecommunications suppliers propose and agree upon standards for third generation networks and UTRAN specifically, and investigate enhanced data rate and radio capacity.
3GPP Technical Specification Group (TSG) RAN has agreed on a work item, referred to as “Uplink Transmit Diversity for High Speed Packet Access (HSPA)—Closed Loop” Feedback determination in Soft Hand Over (SHO) and comprises, according to the agreement, the following items or features:
Precoding weights, which are used to control transmission in e.g. strength and direction, are determined by a single cell.
RNC configures which cell that shall determine the precoding weights for each UE and sends signals to NodeB over NodeB Application Part (NBAP) and to UE over Radio Resource Control (RRC) protocols. NBAP is a signalling protocol responsible for the control of the NodeB,
If an Enhanced-Dedicated Channel (E-DCH) serving cell is configured, this E-DCH serving cell is configured to determine the precoding weights. E-DCH is a transport uplink channel used in the 3G technologies, such as HSPA.
If no E-DCH serving cell is configured, if an High Speed—Downlink Shared Channel (HS-DSCH) serving cell is configured, this cell is configured to determine the precoding weights.
In the case that the UE is not configured with HSPA, there will be no serving cell and Uplink (UL) Closed Loop Transmit Diversity (CLTD) shall still apply. That is direction of transmission and transmission, due to precoding weights, from a plurality of transmit antennas is controlled by feedback from the NodeB. It is up to RNC to decide which cell or NodeB is configured to determine the precoding weights.
High Speed—Shared Control Channel (HS-SCCH) orders are to be used as a way to activate or to deactivate UL CLTD when High Speed Downlink Packet Access (HSDPA) is configured. In the case that the user equipment is not configured with HSDPA, the Serving (S)-RNC-based activation/deactivation of UL CLTD is applicable, i.e., NodeB informs the S-RNC of CLTD activation/deactivation and S-RNC informs the UE.
When there is an E-DCH serving cell, or a HS-DSCH serving cell, the serving cell is used to determine the precoding weights. When there is a handover involving a serving cell change, the new serving cell will perform the above tasks and the old serving cell is aware of the change. When there is a handover not involving a serving cell change, the original serving cell will continue control of UL CLTD, control herein means determine precoding weights e.g. to beamform, i.e. direct, the transmission.
When there are only Dedicated Physical Channel (DPCH) cells, i.e., the UE is not configured with HSPA, SRNC has to decide which cell should be configured to determine the precoding weights, to be the control cell. During soft handover, a new Radio Link is added to a new cell and the new cell may be used as the control cell of UL CLTD. But there is only signalling between S-RNC and the new NodeB serving the new cell, i.e. a target NodeB or cell, during this procedure. There is no signalling between SRNC and the old NodeB, i.e. the NodeB serving the previous serving cell i.e. a source NodeB. Hence the old control cell, i.e. the source cell, will keep the UL CLTD related resources and continue sending UL CLTD related data to the UE at L1/physical interface, even though the UE will be instructed to listen only to the new control cell for UL CLTD. This leads to an ineffective usage of radio resources and may lead to a reduced performance of the radio access network.